<sup>231</sup>Pa and <sup>230</sup>Th in the ocean model of the Community Earth System Model (CESM1.3)

Abstract

Abstract. The sediment 231 Pa / 230 Th activity ratio is emerging as an important proxy for deep ocean circulation in the past. In order to allow for a direct model-data comparison and to improve our understanding of the sediment 231 Pa / 230 Th activity ratio, we implement 231 Pa and 230 Th in the ocean component of the Community Earth System Model (CESM). In addition to the fully coupled implementation of the scavenging behavior of 231 Pa and 230 Th with the active marine ecosystem module (particle-coupled: hereafter p-coupled), another form of 231 Pa and 230 Th have also been implemented with prescribed particle flux fields of the present climate (particle-fixed: hereafter p-fixed). The comparison of the two forms of 231 Pa and 230 Th helps to isolate the influence of the particle fluxes from that of ocean circulation. Under present-day climate forcing, our model is able to simulate water column 231 Pa and 230 Th activity and the sediment 231 Pa / 230 Th activity ratio in good agreement with available observations. In addition, in response to freshwater forcing, the p-coupled and p-fixed sediment 231 Pa / 230 Th activity ratios behave similarly over large areas of low productivity on long timescales, but can differ substantially in some regions of high productivity and on short timescales, indicating the importance of biological productivity in addition to ocean transport. Therefore, our model provides a potentially powerful tool to help the interpretation of sediment 231 Pa / 230 Th reconstructions and to improve our understanding of past ocean circulation and climate changes.

Modeling Neodymium Isotopes in the Ocean Component of the Community Earth System Model (CESM1)

“…However, by comparing results from the p ‐coupled and p ‐fixed Nd setups, we find that the ε Nd changes in our model are mainly controlled by the circulation change. This is different from 231 Pa/ 230 Th in the CESM, which was found to be controlled by both the circulation and the productivity of biogenic particles (Gu & Liu, ). The changes in ε Nd caused by changes in biological productivity are much smaller than the ε Nd changes caused by changes in the circulation (Figures c, f, and g).…”

“…In HOSING, the productivity of CaCO 3 , opal, and POC in the North Atlantic is greatly reduced (Gu & Liu, ) because of reduced nutrient supply resulting from the increase of stratification (Plattner et al, ; Schmittner, ). The resulting particle flux changes could also lead to a change in the p ‐coupled ε Nd (Rempfer et al, ).…”

“…For simplicity and computational efficiency, the particle fluxes can also be prescribed using those generated by the biogeochemical module under modern climate ( p ‐fixed Nd). The reversible scavenging scheme in this study is the same as the scheme applied to model 231 Pa and 230 Th in the CESM (Gu & Liu, ). The CaCO 3 , opal, and POC fluxes produced by the biogeochemical module in the CESM are in reasonable agreement with the observations (Gu & Liu, ).…”

“…The reversible scavenging scheme in this study is the same as the scheme applied to model 231 Pa and 230 Th in the CESM (Gu & Liu, ). The CaCO 3 , opal, and POC fluxes produced by the biogeochemical module in the CESM are in reasonable agreement with the observations (Gu & Liu, ). The dust flux is prescribed using the output from Luo et al ().…”

“…The HOSING experiment is forced by a freshwater flux of 1 Sv to 50–70°N in the North Atlantic. The HOSING experiment is the same experiment as in Gu and Liu (). HOSING is initialized from the end of CTRL and is integrated for 1,400 model years, using the same f boundary and and the Coordinated Ocean‐ice Reference Experiments, phase II, surface forcing as in CTRL.…”